1. Field of the Invention
The present invention relates to a shift control system for a vehicle automatic transmission and, particularly, to a shift control system of the type which comprises engine load sensor means for producing a signal corresponding to an engine load; vehicle speed sensor means for generating a signal corresponding to an actual vehicle speed; means responsive to the signals from the engine load sensor means and the vehicle speed sensor means and a signal indicative of an associated shift position of the automatic transmission for determining a programmed acceleration corresponding to the engine load, the actual vehicle speed and the associated shift position of the automatic transmission; means responsive to the signal from the vehicle speed sensor for producing a signal indicating an actual acceleration; and means responsive to the programmed acceleration and the actual acceleration for selecting a required one from predetermined various relationships (shift patterns) between the engine load and a plurality of reference shifting speeds of the automatic transmission, the reference shifting speed corresponding to the engine load taken from a relationship as selected being compared with the actual speed to decide whether shifting of the automatic transmission is necessary or not.
2. Description of the Prior Art
In known shift control systems of this kind, an output signal of the vehicle speed sensor and an output signal of the engine load sensor, for example, a throttle sensor which provides a signal proportional to a throttle opening, are received continuously or at a given time interval, so that from these data and a signal indicating a shift position of the automatic transmission at that time, a programmed acceleration is determined which is to be attained with the associated shift position, throttle opening, and vehicle speed under given running conditions, for example, a running condition on the road having a slope of 5% and that having a slope of 10%. Then, the programmed acceleration thus attained is compared with an actual acceleration derived from the speed signal provided by the vehicle speed sensor in order to determine continually or at a fixed time interval a desired shift pattern, for example, a running pattern for the flat road with its reference shifting speed being set to be low, a running pattern for the road of a small slope (of larger than or equal to 5%) with its reference shifting speed being set slightly high, or a running pattern for the road of a steep slope (of larger than or equal to 10%) with its reference shifting speed being set to be higher.
However, there arise problems, for example, when the shift pattern is determined continuously as in the former case, the shift control system may respond to changes in acceleration of the vehicle that occur upon its running on an uneven road as shown in FIGS. 1A and 1B, so that the shift pattern is caused to vary unnecessarily, thereby resulting in undesired change of the vehicle speed and disturbing the stable running of the vehicle.
On the other hand, if the signal from the speed sensor is read into an associated storage means at a fixed time interval t.sub.A as indicated by a pointer mark in FIG. 1B so that the shift pattern is determined on the basis of an instantaneous value of an acceleration derived from the speed signal at the fixed time interval, it can be seen from FIGS. 1A and 1B that unnecessary changes of the shift pattern by variations in acceleration of the vehicle due to the unevenness of the road are avoided. In this case, however, operation of the sensors in vehicles with considerable noise, vibration, etc. become astable thereby, and, upon reading data into a memory means at the fixed time interval, incorrect input data may be read into the memory means. Particularly, when a throttle sensor of the potentiometer type is used, it may happen that its closed contacts are accidentally opened because of vibration and even that no input data is read into the memory means. Consequently, there is a sufficient problem that an incorrect shift pattern can be selected based upon erroneous input data as described above, whereby a gear or shift position that is not matched to the running condition of the vehicle is carried out. In particular, such a problem is encountered when input data subject to fluctuations due to vibration of the vehicle is read into the memory means at every fixed time interval in succession, a shift pattern to be selected is also caused to be varied at said every fixed time interval in sequence, which may even lead to a sequence of sudden shifting, for example, sequential shifting of the second speed to the third speed and then to the second speed, so that the vehicle becomes very uncomfortable to ride in. In addition, if an incorrect shift pattern has been chosen, a problem occurs such that said incorrect shift pattern is maintained for a predetermined duration of time. Particularly, if said predetermined duration is increased in order to avoid undesired changes of the shift pattern due to the unevenness of the road as mentioned before, an incorrect shift pattern is caused to be sustained for an excessively long duration.